The present invention relates to an optical information read/write device provided with a plurality of kinds of optical discs having, for example, different recording densities or different protective layer thicknesses, and an objective lens incorporated in the same.
Conventionally, there is a plurality of standards for optical discs such as a CD and a DVD that have different recording densities or different protective layer thicknesses. In addition, recently, in order to attain a higher capacity of information recording, a new standard of optical disc with a still higher recording density than that of the DVD is being brought into practical use. There are cited as the new standard of optical disc an HD DVD, and a BD (Blue-ray Disk). Such a new standard of optical disc has a protective layer whose thickness is equal to or thinner than that of the DVD. In this way, since there is the plurality of optical discs conforming to different standards, an optical information read/write device, more strictly, an objective optical system provided therein is currently required to ensure compatibility among the plurality of optical discs for the sake of user's convenience. It is noted that, in this specification, “optical information read/write devices” include all kinds of devices such as a device used only for reading the information, a device used only for writing the information, and a device used for both of reading and writing the information. Further, “to ensure compatibility” means that read/write operations for writing the information onto the optical disc and/or reading the information therefrom are ensured without exchanging any parts, even when an optical disc on which the operations are to be performed is changed.
In order to attain the device configured to ensure the compatibility among the plurality of optical discs conforming to the different standards, it is necessary to obtain a beam spot with a diameter corresponding to the different recording densities by changing a numerical aperture (NA) for light used in read/write operations for reading or writing the information, compensating spherical aberration that varies depending on a protective layer thickness. In general, the spot diameter of light with a shorter wavelength can be reduced smaller. For this reason, laser beams with different wavelengths are employed for the different recording densities in a conventional optical information read/write device. For example, a laser beam with a wavelength of about 790 nm is applied in use of the CD, while a laser beam with a wavelength of about 660 nm is applied in use of the DVD. In addition, in use of the new standard of optical disc, there is applied a laser beam that has a shorter wavelength than that of the DVD (e.g., a so-called blue laser with a wavelength of about 408 nm).
Moreover, a technique is brought into practical use, in which there is provided on any one surface of at least one optical element (e.g., objective lens) constituting the objective optical system a ring-shaped zone structure having a plurality of microscopic ring-shaped steps to converge each of the light beams with the different wavelengths on a recording surface of a corresponding one of the optical discs under an appropriate condition.
The aforementioned optical element is desired to be able to compensate the spherical aberration due to a wavelength shift, which is caused by the individual difference of a light source or an environmental variation such as a temperature change, from a design wavelength of the laser beam being used. It is noted that the “design wavelength” means a wavelength of each of the laser beams that is optimized for the read/write operations on each of the optical discs.
For example, such an objective lens as to ensure the compatibility among three kinds of optical discs such as the CD, DVD, and HD DVD as aforementioned is proposed in Japanese Patent Provisional Publication No. 2004-247025 (hereinafter, referred to as '025 Publication).
In '025 Publication, there is disclosed an optical pickup device that ensures the compatibility among three kinds of discs with different recording densities. More specifically, an objective lens incorporated in the optical pickup device is provided with a ring-shaped zone structure such that third-order diffracted light is used in read/write operations on an optical disc with a higher recording density, while second-order diffracted light is used in read/write operations on the DVD or the CD. Such an objective lens incorporated in the optical pickup device forms a suitable spot for the read/write operations on a recording surface of each of the optical discs. Thereby, there is provided the optical pickup device that ensures the compatibility among the three kinds of optical discs with the different recording densities.
However, in the optical pickup device disclosed in '025 Publication, only a light usage efficiency of about 40% is obtained in the read/write operations. In addition, unnecessary order diffracted light (first-order diffracted light in this case) is generated. Therefore, a problem arises that a waveform of a focus error signal is so deformed that a focusing function is deteriorated, or the beam spot cannot be converged to a predetermined diameter.